Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider: - [High] drm/gud: missing READ_ONCE() on stats_actual_length in gud_stats_debugfs() --
commit c9d9a9652db7619eedb8feff2da3862c3012fe31 Author: YuanZhe Liu <[email protected]> drm/gud: avoid divide error in gud_stats_debugfs This commit copies gdrm->stats_actual_length into a local variable in gud_stats_debugfs() and skips the compression ratio division when it is zero to avoid a divide-by-zero error before the first framebuffer flush or during a concurrent counter wrap-around reset in gud_flush_rect(). > diff --git a/drivers/gpu/drm/gud/gud_drv.c b/drivers/gpu/drm/gud/gud_drv.c > index 680447f426f37..cec5d64e152bb 100644 > --- a/drivers/gpu/drm/gud/gud_drv.c > +++ b/drivers/gpu/drm/gud/gud_drv.c > @@ -329,10 +329,17 @@ static int gud_stats_debugfs(struct seq_file *m, void > *data) > seq_puts(m, "\n"); > > if (gdrm->compression) { > + u64 actual_length = gdrm->stats_actual_length; > u64 remainder; > - u64 ratio = div64_u64_rem(gdrm->stats_length, > gdrm->stats_actual_length, > - &remainder); > - u64 ratio_frac = div64_u64(remainder * 10, > gdrm->stats_actual_length); > + u64 ratio, ratio_frac; > + > + if (!actual_length) { > + seq_puts(m, "Compression ratio: n/a\n"); > + return 0; > + } > + > + ratio = div64_u64_rem(gdrm->stats_length, actual_length, > &remainder); > + ratio_frac = div64_u64(remainder * 10, actual_length); [Severity: High] Can the compiler reload gdrm->stats_actual_length from memory after the zero check? Neither gud_stats_debugfs() nor gud_flush_damage() -> gud_flush_rect() holds a shared lock when accessing gdrm->stats_actual_length: drivers/gpu/drm/gud/gud_pipe.c:gud_flush_rect() { ... gdrm->stats_length += len; /* Did it wrap around? */ if (gdrm->stats_length <= len && gdrm->stats_actual_length) { gdrm->stats_length = len; gdrm->stats_actual_length = 0; } gdrm->stats_actual_length += trlen; ... } Because actual_length is loaded in gud_stats_debugfs() via a plain C assignment without READ_ONCE(), the compiler is permitted to reload gdrm->stats_actual_length from memory when passing actual_length to div64_u64_rem() or div64_u64(). If gud_flush_rect() concurrently resets gdrm->stats_actual_length to 0, could the zero check observe a non-zero value while the subsequent division reloads 0 and still triggers a divide error? Should gud_stats_debugfs() use READ_ONCE(gdrm->stats_actual_length) (or synchronize the stats counters between gud_flush_rect() and gud_stats_debugfs())? -- Sashiko AI review ยท https://sashiko.dev/#/patchset/[email protected]?part=1
