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Date: Thu, 2 Nov 2017 11:40:35 -0400 (EDT) From: Alan Stern <stern@...land.harvard.edu> To: Peter Zijlstra <peterz@...radead.org> cc: "Reshetova, Elena" <elena.reshetova@...el.com>, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>, "keescook@...omium.org" <keescook@...omium.org>, "tglx@...utronix.de" <tglx@...utronix.de>, "mingo@...hat.com" <mingo@...hat.com>, "ishkamiel@...il.com" <ishkamiel@...il.com>, Will Deacon <will.deacon@....com>, Paul McKenney <paulmck@...ux.vnet.ibm.com>, <parri.andrea@...il.com>, <boqun.feng@...il.com>, <dhowells@...hat.com>, <david@...morbit.com> Subject: Re: [PATCH] refcount: provide same memory ordering guarantees as in atomic_t On Thu, 2 Nov 2017, Peter Zijlstra wrote: > > Lock functions such as refcount_dec_and_lock() & > > refcount_dec_and_mutex_lock() Provide exactly the same guarantees as > > they atomic counterparts. > > Nope. The atomic_dec_and_lock() provides smp_mb() while > refcount_dec_and_lock() merely orders all prior load/store's against all > later load/store's. In fact there is no guaranteed ordering when refcount_dec_and_lock() returns false; it provides ordering only if the return value is true. In which case it provides acquire ordering (thanks to the spin_lock), and both release ordering and a control dependency (thanks to the refcount_dec_and_test). > The difference is subtle and involves at least 3 CPUs. I can't seem to > write up anything simple, keeps turning into monsters :/ Will, Paul, > have you got anything simple around? The combination of acquire + release is not the same as smp_mb, because they allow things to pass by in one direction. Example: C C-refcount-vs-atomic-dec-and-lock { } P0(int *x, int *y, refcount_t *r) { refcount_set(r, 1); WRITE_ONCE(*x, 1); smp_wmb(); WRITE_ONCE(*y, 1); } P1(int *x, int *y, refcount_t *r, spinlock_t *s) { int rx, ry; bool r1; ry = READ_ONCE(*y); r1 = refcount_dec_and_lock(r, s); if (r1) rx = READ_ONCE(*x); } exists (1:ry=1 /\ 1:r1=1 /\ 1:rx=0) This is allowed. The idea is that the CPU can take: Read y Acquire Release Read x and execute the first read after the Acquire and the second read before the Release: Acquire Read y Read x Release and then the CPU can reorder the reads: Acquire Read x Read y Release If the program had used atomic_dec_and_lock() instead, which provides a full smp_mb barrier, this outcome would not be possible. Alan Stern
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