In article <3A0C6BD6.A8F73950@dm.ultramaster.com>,
David Mansfield <lkml@dm.ultramaster.com> wrote:
>Linus Torvalds wrote:
>...
>>
>> And it has everything to do with the fact that the way Linux semaphores
>> are implemented, a non-blocking process has a HUGE advantage over a
>> blocking one. Linux kernel semaphores are extreme unfair in that way.
>>
>...
>> The original running process comes back faulting again, finds the
>> semaphore still unlocked (the "ps" process is awake but has not gotten to
>> run yet), gets the semaphore, and falls asleep on the IO for the next
>> page.
>>
>> The "ps" process actually gets to run now, but it's a bit late. The
>> semaphore is locked again.
>>
>> Repeat until luck breaks the bad circle.
>>
>
>But doesn't __down have a fast path coded in assembly? In other words,
>it only hits your patched code if there is already contention, which
>there isn't in this case, and therefore the bug...?
The __down() case should be hit if there's a waiter, even if that waiter
has not yet been able to pick up the lock (the waiter _will_ have
decremented the count to negative in order to trigger the proper logic
at release time).
But as I mentioned, the pseudo-patch was certainly untested, so
somebody should probably walk through the cases to check that I didn't
miss something.
Linus
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This archive was generated by hypermail 2b29 : Wed Nov 15 2000 - 21:00:19 EST