On Wednesday 09 March 2016 05:10 PM, Peter Zijlstra wrote: > --- > Subject: bitops: Do not default to __clear_bit() for __clear_bit_unlock() > > __clear_bit_unlock() is a special little snowflake. While it carries the > non-atomic '__' prefix, it is specifically documented to pair with > test_and_set_bit() and therefore should be 'somewhat' atomic. > > Therefore the generic implementation of __clear_bit_unlock() cannot use > the fully non-atomic __clear_bit() as a default. > > If an arch is able to do better; is must provide an implementation of > __clear_bit_unlock() itself. > > Specifically, this came up as a result of hackbench livelock'ing in > slab_lock() on ARC with SMP + SLUB + !LLSC. > > The issue was incorrect pairing of atomic ops. > > slab_lock() -> bit_spin_lock() -> test_and_set_bit() > slab_unlock() -> __bit_spin_unlock() -> __clear_bit() > > The non serializing __clear_bit() was getting "lost" > > 80543b8e: ld_s r2,[r13,0] <--- (A) Finds PG_locked is set > 80543b90: or r3,r2,1 <--- (B) other core unlocks right here > 80543b94: st_s r3,[r13,0] <--- (C) sets PG_locked (overwrites unlock) > > Fixes ARC STAR 9000817404 (and probably more). > > Cc: stable at vger.kernel.org > Reported-by: Vineet Gupta <Vineet.Gupta1 at synopsys.com> > Tested-by: Vineet Gupta <Vineet.Gupta1 at synopsys.com> > Signed-off-by: Peter Zijlstra (Intel) <peterz at infradead.org> Peter, I don't see this in linux-next yet. I'm hoping you will send it Linus' way for 4.6-rc1. Thx, -Vineet
