David Miller wrote: > From: "Krzysztof Helt" <krzysztof.h1@xxxxx> > Date: Sat, 27 May 2006 23:09:17 +0200 > >> I tried to build the 2.6.17-rc5 for sparc with SMP enabled. The >> kernel was not build due to a missing macro. Here is a patch >> which fixes this (I copied the macro from a sparc64 header). > > I was going to apply this just to fix the build, but it's > not correct. > > Because we embed a spinlock in the rwlock_t, we have to spin > waiting for the lock bits to be clear before we can trust the > sample of the rest of the lock. We used to have to do this > when we embedded a spinlock into the atomic_t's on sparc32. Correct me if I am wrong, but... >From asm-sparc/spinlock.h: * ------------------------------------ * | 24-bit counter | wlock | raw_rwlock_t * ------------------------------------ * 31 8 7 0 * * wlock signifies the one writer is in or somebody is updating * counter. For a writer, if he successfully acquires the wlock, * but counter is non-zero, he has to release the lock and wait, * till both counter and wlock are zero. rw->lock in this case is the full 32-bit raw_rwlock_t, so a value of 0x000000ff is locked by a writer, 0x00000100 would be locked by a reader, and 0x000001ff would be locked by a reader and updating the counter. For __raw_write_can_lock(), we should be able to return true when the whole 32-bits is zero (i.e. no readers, no counter being updated, and no writers). rw->lock is the whole 32-bits, so a check of !(rw)->lock should be the right thing. While we are at it, another one we are missing is __raw_read_can_lock(), and I think that a check of !((rw)->lock & 0xff) will work (i.e. no writers and no updating of the counter), or might it be better if the counter was negated and we checked rw->lock <= 0 ? Bob - To unsubscribe from this list: send the line "unsubscribe sparclinux" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
