Concurrency primitives, safe memory reclamation mechanisms and non-blocking (including lock-free) data structures designed to aid in the research, design and implementation of high performance concurrent systems developed in C99+.
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This work is from Jonathan T. Looney from the FreeBSD project
(jtl@).
The return value of ck_epoch_poll has also changed. It returns false
only if the epoch counter has not progressed, if no memory was reclaimed
(in other words, no forward progress) and / or not all threads have been
observed in a quiescent state (grace period).
Below are his notes:
Epoch calls are stored in a 4-bucket hash table. The 4-bucket hash table
allows for calls to be stored for three epochs: the current epoch and
two previous ones. The comments at the beginning of ck_epoch.c explain
why this is necessary.
When there are active threads, ck_epoch_poll_deferred() current runs the
epoch calls for the current global epoch + 1. Because of modulo
arithmetic, this is equivalent to running the calls for epoch - 3.
However, this means that ck_epoch_poll_deferred() is waiting
unnecessarily long to run epoch calls.
Further, there could be races in incrementing the global epoch. Imagine
all active threads have observed epoch n. CPU 0 sees this. It increments
the global epoch to (n + 1). It runs the epoch calls for (n - 3). Now,
CPU 1 checks. It sees that there are active threads which have not yet
observed the new global epoch (n + 1). In this case,
ck_epoch_poll_deferred() will return without running any epochs. In the
worst case (CPU 1 continually losing the race), these epoch calls could
be deferred indefinitely.
To fix this, always run any epoch calls for global epoch - 2. Further,
if all active threads have observed the global epoch, run epoch calls
for global epoch - 1.
The global epoch is only incremented when all active threads have
observed it. Therefore, all active threads must always have observed
global epoch - 1 or the current global epoch. Accordingly, it is safe to
always run epoch calls for global epoch - 2.
Further, if all active threads have observed the global epoch, it is
safe to run epoch calls for global epoch - 1.
Samy Al Bahra
dac27da321
