Cc: Andrew Morton <akpm@...>, Austin Clements <amdragon+kernelbugzilla@...>, Ingo Molnar <mingo@...>, john stultz <johnstul@...>, Michael Kerrisk <mtk.manpages@...>, Roland McGrath <roland@...>, Thomas Gleixner <tglx@...>, <linux-kernel@...>
Oh, and the above is *wrong*.
Why?
Becayse if SIGQUEUE_PREALLOC setting needs the lock, then setting any
*other* bit in that word will also need the lock!
That's because
q->flags |= SIGQUEUE_CANCELLED;
writes those other bits too - admittedly with the value they were read
just before, but if it races with something setting SIGQUEUE_PREALLOC that
doesn't matter - the newly written version will simply be wrong.
So the rule is that if one bit of a word needs locking, then they *all*
do.
(On alpha, this is true even for whole bytes or shortwords - because a
byte/shortword write is actually "read word, update byte/short, write
word" sequence on older CPU's. So you cannot do atomic byte updates, and
need to use locks).
Linus
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