Re: When to save/restore_flags() vs cli/sti()

Jes Sorensen (
26 Feb 1999 10:10:06 +0100

>>>>> "Doug" == Doug Ledford <> writes:

Doug> Just becuase I've not seen this mentioned and I'm being anal
Doug> here for people reading this thread thay may not know this.
Doug> save_flags(flags); by itself will not do anything truly
Doug> interesting. You really need this construct:

Doug> save_flags(flags); /* save the current cpu flags */ cli(); /*
Doug> actually turn interrupts off */ ... do stuff
Doug> restore_flags(flags); /* restore the old cpu state */

Ok lets take it one step further then ;-)

On SMP cli(), save_flags() etc. are really expensive as they need to
synchronize across all processors. Therefore using save_flag();cli()
... restore_flags(); in time critical code is a very bad idea.

In the past, before we had spin locks, we used to do it as a quick way
to avoid anybody else fiddling with a data structure while we were
messing with it, however this is now in a way coming back to bite us.

So for time critical code it is a very good idea to look at
spin_lock_irqsave()/spin_unlock_irqrestore() - they will disable
interrupts on the local processor take a spin lock thus not blocking
the other processors until they try to get beyond the lock in
question. For UP machines they will simply turn into the old
save_flags();cli();/restore_flags(); set.


To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to
Please read the FAQ at