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 > <paul...@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 ------------------------------------------------------------------------ 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; -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/