Currently the scope of the raw_write_seqcount_begin/end in sched_clock_register far exceeds the scope of the read section in sched_clock. This gives the impression of safety during cursory review but achieves little.
Note that this is likely to be a latent issue at present because sched_clock_register() is typically called before we enable interrupts, however the issue does risk bugs being needlessly introduced as the code evolves.
This patch fixes the problem by increasing the scope of the read locking performed by sched_clock() to cover all data modified by sched_clock_register.
We also improve clarity by moving writes to struct clock_data that do not impact sched_clock() outside of the critical section.
Signed-off-by: Daniel Thompson daniel.thompson@linaro.org Cc: Russell King linux@arm.linux.org.uk Cc: Will Deacon will.deacon@arm.com Cc: Catalin Marinas catalin.marinas@arm.com Reviewed-by: Stephen Boyd sboyd@codeaurora.org --- kernel/time/sched_clock.c | 25 +++++++++++-------------- 1 file changed, 11 insertions(+), 14 deletions(-)
diff --git a/kernel/time/sched_clock.c b/kernel/time/sched_clock.c index 01d2d15aa662..3d21a8719444 100644 --- a/kernel/time/sched_clock.c +++ b/kernel/time/sched_clock.c @@ -58,23 +58,21 @@ static inline u64 notrace cyc_to_ns(u64 cyc, u32 mult, u32 shift)
unsigned long long notrace sched_clock(void) { - u64 epoch_ns; - u64 epoch_cyc; - u64 cyc; + u64 cyc, res; unsigned long seq;
- if (cd.suspended) - return cd.epoch_ns; - do { seq = raw_read_seqcount_begin(&cd.seq); - epoch_cyc = cd.epoch_cyc; - epoch_ns = cd.epoch_ns; + + res = cd.epoch_ns; + if (!cd.suspended) { + cyc = read_sched_clock(); + cyc = (cyc - cd.epoch_cyc) & sched_clock_mask; + res += cyc_to_ns(cyc, cd.mult, cd.shift); + } } while (read_seqcount_retry(&cd.seq, seq));
- cyc = read_sched_clock(); - cyc = (cyc - epoch_cyc) & sched_clock_mask; - return epoch_ns + cyc_to_ns(cyc, cd.mult, cd.shift); + return res; }
/* @@ -124,10 +122,11 @@ void __init sched_clock_register(u64 (*read)(void), int bits, clocks_calc_mult_shift(&new_mult, &new_shift, rate, NSEC_PER_SEC, 3600);
new_mask = CLOCKSOURCE_MASK(bits); + cd.rate = rate;
/* calculate how many ns until we wrap */ wrap = clocks_calc_max_nsecs(new_mult, new_shift, 0, new_mask); - new_wrap_kt = ns_to_ktime(wrap - (wrap >> 3)); + cd.wrap_kt = ns_to_ktime(wrap - (wrap >> 3));
/* update epoch for new counter and update epoch_ns from old counter*/ new_epoch = read(); @@ -138,8 +137,6 @@ void __init sched_clock_register(u64 (*read)(void), int bits, raw_write_seqcount_begin(&cd.seq); read_sched_clock = read; sched_clock_mask = new_mask; - cd.rate = rate; - cd.wrap_kt = new_wrap_kt; cd.mult = new_mult; cd.shift = new_shift; cd.epoch_cyc = new_epoch;