Re: [PATCH v1 1/1] xarray: fix the data-race in xas_find_chunk() by using READ_ONCE()
From: Jan Kara
Date: Mon Sep 18 2023 - 12:54:49 EST
On Mon 18-09-23 14:46:02, Mirsad Todorovac wrote:
> On 9/18/23 13:38, Jan Kara wrote:
> > On Mon 18-09-23 12:20:09, Mirsad Todorovac wrote:
> > > On 9/18/23 11:41, Jan Kara wrote:
> > > > On Mon 18-09-23 06:47:40, Mirsad Goran Todorovac wrote:
> > > > > KCSAN has discovered the following data-race:
> > > > >
> > > > > [ 206.510010] ==================================================================
> > > > > [ 206.510035] BUG: KCSAN: data-race in xas_clear_mark / xas_find_marked
> > > > >
> > > > > [ 206.510067] write to 0xffff963df6a90fe0 of 8 bytes by interrupt on cpu 22:
> > > > > [ 206.510081] xas_clear_mark (./arch/x86/include/asm/bitops.h:178 ./include/asm-generic/bitops/instrumented-non-atomic.h:115 lib/xarray.c:102 lib/xarray.c:914)
> > > > > [ 206.510097] __xa_clear_mark (lib/xarray.c:1923)
> > > > > [ 206.510114] __folio_end_writeback (mm/page-writeback.c:2981)
> > > > > [ 206.510128] folio_end_writeback (mm/filemap.c:1616)
> > > > > [ 206.510143] end_page_writeback (mm/folio-compat.c:28)
> > > > > [ 206.510155] btrfs_page_clear_writeback (fs/btrfs/subpage.c:646) btrfs
> > > > > [ 206.510994] end_bio_extent_writepage (./include/linux/bio.h:84 fs/btrfs/extent_io.c:542) btrfs
> > > > > [ 206.511817] __btrfs_bio_end_io (fs/btrfs/bio.c:117 fs/btrfs/bio.c:112) btrfs
> > > > > [ 206.512640] btrfs_orig_bbio_end_io (fs/btrfs/bio.c:164) btrfs
> > > > > [ 206.513497] btrfs_simple_end_io (fs/btrfs/bio.c:380) btrfs
> > > > > [ 206.514350] bio_endio (block/bio.c:1617)
> > > > > [ 206.514362] blk_mq_end_request_batch (block/blk-mq.c:837 block/blk-mq.c:1073)
> > > > > [ 206.514377] nvme_pci_complete_batch (drivers/nvme/host/pci.c:986) nvme
> > > > > [ 206.514437] nvme_irq (drivers/nvme/host/pci.c:1086) nvme
> > > > > [ 206.514500] __handle_irq_event_percpu (kernel/irq/handle.c:158)
> > > > > [ 206.514517] handle_irq_event (kernel/irq/handle.c:195 kernel/irq/handle.c:210)
> > > > > [ 206.514533] handle_edge_irq (kernel/irq/chip.c:836)
> > > > > [ 206.514549] __common_interrupt (./include/linux/irqdesc.h:161 arch/x86/kernel/irq.c:238 arch/x86/kernel/irq.c:257)
> > > > > [ 206.514563] common_interrupt (arch/x86/kernel/irq.c:247 (discriminator 14))
> > > > > [ 206.514583] asm_common_interrupt (./arch/x86/include/asm/idtentry.h:636)
> > > > > [ 206.514599] kcsan_setup_watchpoint (kernel/kcsan/core.c:705 (discriminator 1))
> > > > > [ 206.514612] __tsan_read8 (kernel/kcsan/core.c:1025)
> > > > > [ 206.514626] steal_from_bitmap.part.0 (./include/linux/find.h:186 fs/btrfs/free-space-cache.c:2557 fs/btrfs/free-space-cache.c:2613) btrfs
> > > > > [ 206.515491] __btrfs_add_free_space (fs/btrfs/free-space-cache.c:2689 fs/btrfs/free-space-cache.c:2667) btrfs
> > > > > [ 206.516361] btrfs_add_free_space_async_trimmed (fs/btrfs/free-space-cache.c:2798) btrfs
> > > > > [ 206.517231] add_new_free_space (fs/btrfs/block-group.c:550) btrfs
> > > > > [ 206.518095] load_free_space_tree (fs/btrfs/free-space-tree.c:1595 fs/btrfs/free-space-tree.c:1658) btrfs
> > > > > [ 206.518953] caching_thread (fs/btrfs/block-group.c:873) btrfs
> > > > > [ 206.519800] btrfs_work_helper (fs/btrfs/async-thread.c:314) btrfs
> > > > > [ 206.520643] process_one_work (kernel/workqueue.c:2600)
> > > > > [ 206.520658] worker_thread (./include/linux/list.h:292 kernel/workqueue.c:2752)
> > > > > [ 206.520672] kthread (kernel/kthread.c:389)
> > > > > [ 206.520684] ret_from_fork (arch/x86/kernel/process.c:145)
> > > > > [ 206.520701] ret_from_fork_asm (arch/x86/entry/entry_64.S:312)
> > > > >
> > > > > [ 206.520722] read to 0xffff963df6a90fe0 of 8 bytes by task 2793 on cpu 6:
> > > > > [ 206.520735] xas_find_marked (./include/linux/xarray.h:1706 lib/xarray.c:1354)
> > > > > [ 206.520750] filemap_get_folios_tag (mm/filemap.c:1975 mm/filemap.c:2273)
> > > > > [ 206.520763] __filemap_fdatawait_range (mm/filemap.c:519)
> > > > > [ 206.520777] filemap_fdatawait_range (mm/filemap.c:556)
> > > > > [ 206.520790] btrfs_wait_ordered_range (fs/btrfs/ordered-data.c:839) btrfs
> > > > > [ 206.521641] btrfs_sync_file (fs/btrfs/file.c:1859) btrfs
> > > > > [ 206.522495] vfs_fsync_range (fs/sync.c:188)
> > > > > [ 206.522509] __x64_sys_fsync (./include/linux/file.h:45 fs/sync.c:213 fs/sync.c:220 fs/sync.c:218 fs/sync.c:218)
> > > > > [ 206.522522] do_syscall_64 (arch/x86/entry/common.c:50 arch/x86/entry/common.c:80)
> > > > > [ 206.522535] entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:120)
> > > > >
> > > > > [ 206.522557] value changed: 0xfffffffffff80000 -> 0xfffffffffff00000
> > > > >
> > > > > [ 206.522574] Reported by Kernel Concurrency Sanitizer on:
> > > > > [ 206.522585] CPU: 6 PID: 2793 Comm: tracker-extract Tainted: G L 6.5.0-rc6+ #44
> > > > > [ 206.522600] Hardware name: ASRock X670E PG Lightning/X670E PG Lightning, BIOS 1.21 04/26/2023
> > > > > [ 206.522608] ==================================================================
> > > >
> > > > Thanks for working on this. I guess the full KCSAN warning isn't that
> > > > useful in the changelog. Rather I'd spend more time explaining the real
> > > > problem here ...
> > > >
> > > > > As Jan Kara explained, the problem is in the function xas_find_chuck():
> > > > >
> > > > > /* Private */
> > > > > static inline unsigned int xas_find_chunk(struct xa_state *xas, bool advance,
> > > > > xa_mark_t mark)
> > > > > {
> > > > > unsigned long *addr = xas->xa_node->marks[(__force unsigned)mark];
> > > > > unsigned int offset = xas->xa_offset;
> > > > >
> > > > > if (advance)
> > > > > offset++;
> > > > > if (XA_CHUNK_SIZE == BITS_PER_LONG) {
> > > > > if (offset < XA_CHUNK_SIZE) {
> > > > > → unsigned long data = *addr & (~0UL << offset);
> > > > > if (data)
> > > > > return __ffs(data);
> > > >
> > > > ... which is that xas_find_chunk() is called only under RCU protection and
> > > > thus the two uses of 'data' in the above code can yield different results.
> > > >
> > > > > }
> > > > > return XA_CHUNK_SIZE;
> > > > > }
> > > > >
> > > > > return find_next_bit(addr, XA_CHUNK_SIZE, offset);
> > > > > }
> > > > >
> > > > > In particular, the line
> > > > >
> > > > > unsigned long data = *addr & (~0UL << offset);
> > > > >
> > > > > contains a data race that is best avoided using READ_ONCE(), which eliminated the KCSAN
> > > > > data-race warning completely.
> > > >
> > > > Yes, this improves the situation for xarray use on 64-bit architectures but
> > > > doesn't fix cases on 32-bit archs or if CONFIG_BASE_SMALL is set. As I
> > > > mentioned in my previous reply, I'd rather:
> > > >
> > > > 1) Fix find_next_bit(), find_first_bit() and related functions in
> > > > lib/find_bit.c to use READ_ONCE() - such as _find_first_bit() etc. It is
> > > > quite some churn but I don't see how else to make these functions safe when
> > > > the underlying contents can change.
> > >
> > > Thank you for your review.
> > >
> > > I assume you have the big picture, but just a stupid question:
> > >
> > > if (XA_CHUNK_SIZE == BITS_PER_LONG) {
> > > if (offset < XA_CHUNK_SIZE) {
> > > unsigned long data = READ_ONCE(*addr) & (~0UL << offset);
> > > if (data)
> > > return __ffs(data);
> > > }
> > > return XA_CHUNK_SIZE;
> > > }
> > >
> > > I would hate to argue, but ...
> >
> > No problem, asking questions isn't argueing ;).
> >
> > > Wouldn't BITS_PER_LONG simply change to 32 on 32-bit architectures?
> >
> > Yes, they will. But XA_CHUNK_SIZE will still be 64 on 32-bit AFAICT so
> > XA_CHUNK_SIZE != BITS_PER_LONG there.
>
> Ah, I see. This is definitely not good. But I managed to fix and test the find_next_bit()
> family, but this seems that simply
>
> -------------------------------------------
> include/linux/xarray.h | 8 --------
> 1 file changed, 8 deletions(-)
>
> diff --git a/include/linux/xarray.h b/include/linux/xarray.h
> index 1715fd322d62..89918b65b00d 100644
> --- a/include/linux/xarray.h
> +++ b/include/linux/xarray.h
> @@ -1718,14 +1718,6 @@ static inline unsigned int xas_find_chunk(struct xa_state *xas, bool advance,
> if (advance)
> offset++;
> - if (XA_CHUNK_SIZE == BITS_PER_LONG) {
> - if (offset < XA_CHUNK_SIZE) {
> - unsigned long data = READ_ONCE(*addr) & (~0UL << offset);
> - if (data)
> - return __ffs(data);
> - }
> - return XA_CHUNK_SIZE;
> - }
> return find_next_bit(addr, XA_CHUNK_SIZE, offset);
> }
>
> seems too good to be true.
>
> According to what you explained, the performance impact would be negligent or non-existing,
> and the CONFIG_BASE_SMALL problem would disappear?
If you fix find_next_bit(), then this will fix xas_find_chunk(), yes.
> I did not even try to run that, as I am not 100% confident in the logic.
>
> Am I doing something very wrong?
No, this is exactly what I think needs to happen.
> > > Is there something I am missing?
> > >
> > > From include/asm-generic/bitsperlong.h:
> > > ----------------------------------------
> > > #ifdef CONFIG_64BIT
> > > #define BITS_PER_LONG 64
> > > #else
> > > #define BITS_PER_LONG 32
> > > #endif /* CONFIG_64BIT */
> > >
> > > About the CONFIG_BASE_SMALL I cannot tell:
> > > ----------------------------------------
> > > #ifndef XA_CHUNK_SHIFT
> > > #define XA_CHUNK_SHIFT (CONFIG_BASE_SMALL ? 4 : 6)
> > > #endif
> > > #define XA_CHUNK_SIZE (1UL << XA_CHUNK_SHIFT)
> > > #define XA_CHUNK_MASK (XA_CHUNK_SIZE - 1)
> > > #define XA_MAX_MARKS 3
> > > #define XA_MARK_LONGS DIV_ROUND_UP(XA_CHUNK_SIZE, BITS_PER_LONG)
> > > ----------------------------------------
> >
> > Again with CONFIG_BASE_SMALL we have XA_CHUNK_SIZE == 16 so it will not be
> > equal to BITS_PER_LONG.
> > > I see why you would want find_next_bit() and find_first_bit() fixed, but
> > > I am not that deep into those bitops, so I guess I cannot make this in
> > > one step ... Probably it would require a lot of homework.
> > >
> > > _find_*_bit() functions and/or macros cause quite a number of KCSAN BUG warnings:
> > >
> > > 95 _find_first_and_bit (lib/find_bit.c:114 (discriminator 10))
> > > 31 _find_first_zero_bit (lib/find_bit.c:125 (discriminator 10))
> > > 173 _find_next_and_bit (lib/find_bit.c:171 (discriminator 2))
> > > 655 _find_next_bit (lib/find_bit.c:133 (discriminator 2))
> > > 5 _find_next_zero_bit
> > >
> > > ... but I am simply not certain what is the right thing to do ATM about
> > > those and whether they are false positives.
> >
> > Well, it would require some auditing to be sure but there is at least one
> > user of these functions (xarray) where the problem is real so given the fix
> > has no real runtime cost the fix looks justified.
> >
> > > AFAICS, READ_ONCE() here solves the case of 64 and 32 architectures which is
> > > an incremental step, and it works ... I am just not ready for an
> > > universal solution ATM.
> > >
> > > > 2) Change xas_find_chunk() to unconditionally use find_next_bit() as the
> > > > special case XA_CHUNK_SIZE == BITS_PER_LONG seems pointless these days
> > > > because find_next_bit() is inline and does small_const_nbits(size) check.
> > >
> > > I see your point. A generalised solution would of course be better. But
> > > from the report about data-races in those functions it seems that they
> > > need a major rethink. It isn't that obvious to me what should be
> > > READ_ONCE()-ed in a bit field ...
> >
> > Well, it's actually not that difficult. They all need a treatment like:
> >
> > unsigned long _find_next_bit(const unsigned long *addr, unsigned long nbits, uns
> > {
> > - return FIND_NEXT_BIT(addr[idx], /* nop */, nbits, start);
> > + return FIND_NEXT_BIT(READ_ONCE(addr[idx]), /* nop */, nbits, start);
> > }
> >
> >
> > > Those functions are extensively used throughout the kernel and I get the
> > > notion it is a job for someone with more experience ...
> >
> > Sure, if you don't feel like doing the general change, I can look into it
> > myself.
> >
> > Honza
>
> Hi,
>
> I tried this patch and the
>
> > > 95 _find_first_and_bit (lib/find_bit.c:114 (discriminator 10))
> > > 31 _find_first_zero_bit (lib/find_bit.c:125 (discriminator 10))
> > > 173 _find_next_and_bit (lib/find_bit.c:171 (discriminator 2))
> > > 655 _find_next_bit (lib/find_bit.c:133 (discriminator 2))
> > > 5 _find_next_zero_bit
>
> data-races do not seem to appear any longer.
Yup. You've just missed one case in _find_last_bit() and then all the
functions in include/linux/find.h need a similar treatment...
Honza
>
> --------------------------------------------------------
> lib/find_bit.c | 33 +++++++++++++++++----------------
> 1 file changed, 17 insertions(+), 16 deletions(-)
>
> diff --git a/lib/find_bit.c b/lib/find_bit.c
> index 32f99e9a670e..56244e4f744e 100644
> --- a/lib/find_bit.c
> +++ b/lib/find_bit.c
> @@ -18,6 +18,7 @@
> #include <linux/math.h>
> #include <linux/minmax.h>
> #include <linux/swab.h>
> +#include <asm/rwonce.h>
> /*
> * Common helper for find_bit() function family
> @@ -98,7 +99,7 @@ out: \
> */
> unsigned long _find_first_bit(const unsigned long *addr, unsigned long size)
> {
> - return FIND_FIRST_BIT(addr[idx], /* nop */, size);
> + return FIND_FIRST_BIT(READ_ONCE(addr[idx]), /* nop */, size);
> }
> EXPORT_SYMBOL(_find_first_bit);
> #endif
> @@ -111,7 +112,7 @@ unsigned long _find_first_and_bit(const unsigned long *addr1,
> const unsigned long *addr2,
> unsigned long size)
> {
> - return FIND_FIRST_BIT(addr1[idx] & addr2[idx], /* nop */, size);
> + return FIND_FIRST_BIT(READ_ONCE(addr1[idx]) & READ_ONCE(addr2[idx]), /* nop */, size);
> }
> EXPORT_SYMBOL(_find_first_and_bit);
> #endif
> @@ -122,7 +123,7 @@ EXPORT_SYMBOL(_find_first_and_bit);
> */
> unsigned long _find_first_zero_bit(const unsigned long *addr, unsigned long size)
> {
> - return FIND_FIRST_BIT(~addr[idx], /* nop */, size);
> + return FIND_FIRST_BIT(~READ_ONCE(addr[idx]), /* nop */, size);
> }
> EXPORT_SYMBOL(_find_first_zero_bit);
> #endif
> @@ -130,28 +131,28 @@ EXPORT_SYMBOL(_find_first_zero_bit);
> #ifndef find_next_bit
> unsigned long _find_next_bit(const unsigned long *addr, unsigned long nbits, unsigned long start)
> {
> - return FIND_NEXT_BIT(addr[idx], /* nop */, nbits, start);
> + return FIND_NEXT_BIT(READ_ONCE(addr[idx]), /* nop */, nbits, start);
> }
> EXPORT_SYMBOL(_find_next_bit);
> #endif
> unsigned long __find_nth_bit(const unsigned long *addr, unsigned long size, unsigned long n)
> {
> - return FIND_NTH_BIT(addr[idx], size, n);
> + return FIND_NTH_BIT(READ_ONCE(addr[idx]), size, n);
> }
> EXPORT_SYMBOL(__find_nth_bit);
> unsigned long __find_nth_and_bit(const unsigned long *addr1, const unsigned long *addr2,
> unsigned long size, unsigned long n)
> {
> - return FIND_NTH_BIT(addr1[idx] & addr2[idx], size, n);
> + return FIND_NTH_BIT(READ_ONCE(addr1[idx]) & READ_ONCE(addr2[idx]), size, n);
> }
> EXPORT_SYMBOL(__find_nth_and_bit);
> unsigned long __find_nth_andnot_bit(const unsigned long *addr1, const unsigned long *addr2,
> unsigned long size, unsigned long n)
> {
> - return FIND_NTH_BIT(addr1[idx] & ~addr2[idx], size, n);
> + return FIND_NTH_BIT(READ_ONCE(addr1[idx]) & ~READ_ONCE(addr2[idx]), size, n);
> }
> EXPORT_SYMBOL(__find_nth_andnot_bit);
> @@ -160,7 +161,7 @@ unsigned long __find_nth_and_andnot_bit(const unsigned long *addr1,
> const unsigned long *addr3,
> unsigned long size, unsigned long n)
> {
> - return FIND_NTH_BIT(addr1[idx] & addr2[idx] & ~addr3[idx], size, n);
> + return FIND_NTH_BIT(READ_ONCE(addr1[idx]) & READ_ONCE(addr2[idx]) & ~READ_ONCE(addr3[idx]), size, n);
> }
> EXPORT_SYMBOL(__find_nth_and_andnot_bit);
> @@ -168,7 +169,7 @@ EXPORT_SYMBOL(__find_nth_and_andnot_bit);
> unsigned long _find_next_and_bit(const unsigned long *addr1, const unsigned long *addr2,
> unsigned long nbits, unsigned long start)
> {
> - return FIND_NEXT_BIT(addr1[idx] & addr2[idx], /* nop */, nbits, start);
> + return FIND_NEXT_BIT(READ_ONCE(addr1[idx]) & READ_ONCE(addr2[idx]), /* nop */, nbits, start);
> }
> EXPORT_SYMBOL(_find_next_and_bit);
> #endif
> @@ -177,7 +178,7 @@ EXPORT_SYMBOL(_find_next_and_bit);
> unsigned long _find_next_andnot_bit(const unsigned long *addr1, const unsigned long *addr2,
> unsigned long nbits, unsigned long start)
> {
> - return FIND_NEXT_BIT(addr1[idx] & ~addr2[idx], /* nop */, nbits, start);
> + return FIND_NEXT_BIT(READ_ONCE(addr1[idx]) & ~READ_ONCE(addr2[idx]), /* nop */, nbits, start);
> }
> EXPORT_SYMBOL(_find_next_andnot_bit);
> #endif
> @@ -186,7 +187,7 @@ EXPORT_SYMBOL(_find_next_andnot_bit);
> unsigned long _find_next_or_bit(const unsigned long *addr1, const unsigned long *addr2,
> unsigned long nbits, unsigned long start)
> {
> - return FIND_NEXT_BIT(addr1[idx] | addr2[idx], /* nop */, nbits, start);
> + return FIND_NEXT_BIT(READ_ONCE(addr1[idx]) | READ_ONCE(addr2[idx]), /* nop */, nbits, start);
> }
> EXPORT_SYMBOL(_find_next_or_bit);
> #endif
> @@ -195,7 +196,7 @@ EXPORT_SYMBOL(_find_next_or_bit);
> unsigned long _find_next_zero_bit(const unsigned long *addr, unsigned long nbits,
> unsigned long start)
> {
> - return FIND_NEXT_BIT(~addr[idx], /* nop */, nbits, start);
> + return FIND_NEXT_BIT(~READ_ONCE(addr[idx]), /* nop */, nbits, start);
> }
> EXPORT_SYMBOL(_find_next_zero_bit);
> #endif
> @@ -208,7 +209,7 @@ unsigned long _find_last_bit(const unsigned long *addr, unsigned long size)
> unsigned long idx = (size-1) / BITS_PER_LONG;
> do {
> - val &= addr[idx];
> + val &= READ_ONCE(addr[idx]);
> if (val)
> return idx * BITS_PER_LONG + __fls(val);
> @@ -242,7 +243,7 @@ EXPORT_SYMBOL(find_next_clump8);
> */
> unsigned long _find_first_zero_bit_le(const unsigned long *addr, unsigned long size)
> {
> - return FIND_FIRST_BIT(~addr[idx], swab, size);
> + return FIND_FIRST_BIT(~READ_ONCE(addr[idx]), swab, size);
> }
> EXPORT_SYMBOL(_find_first_zero_bit_le);
> @@ -252,7 +253,7 @@ EXPORT_SYMBOL(_find_first_zero_bit_le);
> unsigned long _find_next_zero_bit_le(const unsigned long *addr,
> unsigned long size, unsigned long offset)
> {
> - return FIND_NEXT_BIT(~addr[idx], swab, size, offset);
> + return FIND_NEXT_BIT(~READ_ONCE(addr[idx]), swab, size, offset);
> }
> EXPORT_SYMBOL(_find_next_zero_bit_le);
> #endif
> @@ -261,7 +262,7 @@ EXPORT_SYMBOL(_find_next_zero_bit_le);
> unsigned long _find_next_bit_le(const unsigned long *addr,
> unsigned long size, unsigned long offset)
> {
> - return FIND_NEXT_BIT(addr[idx], swab, size, offset);
> + return FIND_NEXT_BIT(READ_ONCE(addr[idx]), swab, size, offset);
> }
> EXPORT_SYMBOL(_find_next_bit_le);
> --
--
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR