On Wed, 21 Mar 2018, Jens Axboe wrote: > On 3/21/18 10:42 AM, Mikulas Patocka wrote: > > Early alpha processors cannot write a single byte or word; they read 8 > > bytes, modify the value in registers and write back 8 bytes. > > > > The type blk_status_t is defined as one byte, it is often written > > asynchronously by I/O completion routines, this asynchronous modification > > can corrupt content of nearby bytes if these nearby bytes can be written > > simultaneously by another CPU. > > > > - one example of such corruption is the structure dm_io where > > "blk_status_t status" is written by an asynchronous completion routine > > and "atomic_t io_count" is modified synchronously > > - another example is the structure dm_buffer where "unsigned hold_count" > > is modified synchronously from process context and "blk_status_t > > write_error" is modified asynchronously from bio completion routine > > > > This patch fixes the bug by changing the type blk_status_t to 32 bits if > > we are on Alpha and if we are compiling for a processor that doesn't have > > the byte-word-extension. > > That's nasty. Is alpha the only problematic arch here? Yes. All the other architectures supported by Linux have byte writes. > As to the patch in question, normally I'd just say we should make it > unconditionally u32. But we pack so nicely in the bio, and I don't think > the bio itself has this issue as the rest of the members that share this > word are all set before the bio is submitted. But callers embedding > the status var in other structures don't necessarily have that > guarantee, as your dm examples show. > > -- > Jens Axboe Keeping blk_status_t 8-bit for most architectures will save a few bytes in some of device mapper structures. Mikulas -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
