Messages in this thread | | | Date | Sun, 19 Jan 2014 16:52:15 -0800 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH v8 4/4] qrwlock: Use smp_store_release() in write_unlock() |
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On Sun, Jan 19, 2014 at 11:56:02AM -0800, Linus Torvalds wrote: > On Sun, Jan 19, 2014 at 12:04 AM, Paul E. McKenney > <paulmck@linux.vnet.ibm.com> wrote: > > > > OK, another approach would be to never add "select ARCH_USE_QUEUE_RWLOCK" > > on Alpha, at least if the queued rwlocks really do want to atomically > > manipulate bytes. After all, the Alpha systems that I know about don't > > have enough CPUs to make queued rwlocks necessary anyway. > > > > Much simpler solution! > > > > Is this what you were getting at, or am I missing your point? > > You're missing something. > > Just make the "writer" field be an "int" on little-endian archiectures > (like alpha). > > There is no reason for that field to be a "char" to begin with, as far > as I can tell, since the padding of the structure means that it > doesn't save any space. But even if that wasn't true, we could make an > arch-specific type for "minimum type for locking".
On 64-bit systems (which includes Alpha), agreed, the field can be a 32-bit portion of a 64-bit structure that is then manipulated atomically. Many 32-bit systems need the reader and writer counts to fix in 32 bits in order to allow things like queue_read_trylock() to correctly account for both readers and writers.
If there was a 32-bit system running Linux that did not support byte accesses, there would be a problem, but I don't know of any such system.
> So my *point* was that it should be easy enough to just make sure that > any data structures used for locking have types that are appropriate > for that locking.
So something like the following for the qrwlock definition, with appropriate C-preprocessor wrappers for the atomic-add accesses?
Thanx, Paul
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typedef struct qrwlock { union qrwcnts { #ifdef CONFIG_64B struct ( int writer; int reader; }; atomic_long_t rwa; u64 rwc; #else struct { #ifdef __LITTLE_ENDIAN u8 writer; /* Writer state */ #else u16 r16; /* Reader count - msb */ u8 r8; /* Reader count - lsb */ u8 writer; /* Writer state */ #endif }; atomic_t rwa; /* Reader/writer atomic */ u32 rwc; /* Reader/writer counts */ } cnts; #endif struct mcs_spinlock *waitq; /* Tail of waiting queue */ } arch_rwlock_t;
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