+CC linux-arch, parisc folks, PeterZ On Wednesday 09 March 2016 02:10 AM, Christoph Lameter wrote: > On Tue, 8 Mar 2016, Vineet Gupta wrote: > >> # set the bit >> 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) > > Duh. Guess you need to take the spinlock also in the arch specific > implementation of __bit_spin_unlock(). This is certainly not the only case > in which we use the __ op to unlock. __bit_spin_lock() by definition is *not* required to be atomic, bit_spin_lock() is - so I don't think we need a spinlock there. There is clearly a problem in slub code that it is pairing a test_and_set_bit() with a __clear_bit(). Latter can obviously clobber former if they are not a single instruction each unlike x86 or they use llock/scond kind of instructions where the interim store from other core is detected and causes a retry of whole llock/scond sequence. BTW ARC is not the only arch which suffers from this - other arches potentially also are. AFAIK PARISC also doesn't have atomic r-m-w and also uses a set of external hashed spinlocks to protect the r-m-w sequences. https://lkml.org/lkml/2014/6/1/178 So there also we have the same race because the outer spin lock is not taken for slab_unlock() -> __bit_spin_lock() -> __clear_bit. Arguably I can fix the ARC !LLSC variant of test_and_set_bit() to not set the bit unconditionally but only if it was clear (PARISC does the same). That would be a slight micro-optimization as we won't need another snoop transaction to make line writable and that would also elide this problem, but I think there is a fundamental problem here in slub which is pairing atomic and non atomic ops - for performance reasons. It doesn't work on all arches and/or configurations. > You need a true atomic op or you need to take the "spinlock" in all > cases where you modify the bit. No we don't in __bit_spin_lock and we already do in bit_spin_lock. > If you take the lock in __bit_spin_unlock > then the race cannot happen. Of course it won't but that means we penalize all non atomic callers of the API with a superfluous spinlock which is not require din first place given the definition of API. >> Are you convinced now ! > > Yes, please fix your arch specific code. -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
