From d2164409343445e1f2de62b210218ad05c75d4c9 Mon Sep 17 00:00:00 2001 From: Craig Tiller Date: Mon, 20 Mar 2017 06:45:39 -0700 Subject: Switch to no-barrier loads + full barrier cas-s to solve ABA problem --- src/core/lib/iomgr/ev_epoll_linux.c | 115 +++++++++++++----------------------- 1 file changed, 41 insertions(+), 74 deletions(-) (limited to 'src/core/lib/iomgr/ev_epoll_linux.c') diff --git a/src/core/lib/iomgr/ev_epoll_linux.c b/src/core/lib/iomgr/ev_epoll_linux.c index 11208b9ad1..f1e14e9868 100644 --- a/src/core/lib/iomgr/ev_epoll_linux.c +++ b/src/core/lib/iomgr/ev_epoll_linux.c @@ -1107,19 +1107,15 @@ static void fd_orphan(grpc_exec_ctx *exec_ctx, grpc_fd *fd, static void notify_on(grpc_exec_ctx *exec_ctx, grpc_fd *fd, gpr_atm *state, grpc_closure *closure) { while (true) { - /* Fast-path: CLOSURE_NOT_READY -> . - The 'release' cas here matches the 'acquire' load in set_ready and - set_shutdown ensuring that the closure (scheduled by set_ready or - set_shutdown) happens-after the I/O event on the fd */ - if (gpr_atm_rel_cas(state, CLOSURE_NOT_READY, (gpr_atm)closure)) { - return; /* Fast-path successful. Return */ - } - - /* Slowpath. The 'acquire' load matches the 'release' cas in set_ready and - set_shutdown */ - gpr_atm curr = gpr_atm_acq_load(state); + gpr_atm curr = gpr_atm_no_barrier_load(state); switch (curr) { case CLOSURE_NOT_READY: { + /* CLOSURE_NOT_READY -> . */ + if (gpr_atm_no_barrier_cas(state, CLOSURE_NOT_READY, + (gpr_atm)closure)) { + return; /* Successful. Return */ + } + break; /* retry */ } @@ -1165,30 +1161,19 @@ static void notify_on(grpc_exec_ctx *exec_ctx, grpc_fd *fd, gpr_atm *state, static void set_shutdown(grpc_exec_ctx *exec_ctx, grpc_fd *fd, gpr_atm *state, grpc_error *shutdown_err) { - /* Try the fast-path first (i.e expect the current value to be - CLOSURE_NOT_READY */ - gpr_atm curr = CLOSURE_NOT_READY; gpr_atm new_state = (gpr_atm)shutdown_err | FD_SHUTDOWN_BIT; while (true) { - /* The 'release' cas here matches the 'acquire' load in notify_on to ensure - that the closure it schedules 'happens-after' the set_shutdown is called - on the fd */ - if (gpr_atm_rel_cas(state, curr, new_state)) { - return; /* Fast-path successful. Return */ - } - /* Fallback to slowpath. This 'acquire' load matches the 'release' cas in notify_on and set_ready */ - curr = gpr_atm_acq_load(state); + gpr_atm curr = gpr_atm_no_barrier_load(state); switch (curr) { - case CLOSURE_READY: { - break; /* retry */ - } - - case CLOSURE_NOT_READY: { + case CLOSURE_READY: + case CLOSURE_NOT_READY: + if (gpr_atm_full_cas(state, curr, new_state)) { + return; /* early out */ + } break; /* retry */ - } default: { /* 'curr' is either a closure or the fd is already shutdown */ @@ -1199,10 +1184,8 @@ static void set_shutdown(grpc_exec_ctx *exec_ctx, grpc_fd *fd, gpr_atm *state, } /* Fd is not shutdown. Schedule the closure and move the state to - shutdown state. The 'release' cas here matches the 'acquire' load in - notify_on to ensure that the closure it schedules 'happens-after' - the set_shutdown is called on the fd */ - if (gpr_atm_rel_cas(state, curr, new_state)) { + shutdown state. */ + if (gpr_atm_full_cas(state, curr, new_state)) { grpc_closure_sched( exec_ctx, (grpc_closure *)curr, GRPC_ERROR_CREATE_REFERENCING("FD Shutdown", &shutdown_err, 1)); @@ -1220,52 +1203,36 @@ static void set_shutdown(grpc_exec_ctx *exec_ctx, grpc_fd *fd, gpr_atm *state, } static void set_ready(grpc_exec_ctx *exec_ctx, grpc_fd *fd, gpr_atm *state) { - /* Try an optimistic case first (i.e assume current state is - CLOSURE_NOT_READY). - - This 'release' cas matches the 'acquire' load in notify_on ensuring that - any closure (scheduled by notify_on) 'happens-after' the return from - epoll_pwait */ - if (gpr_atm_rel_cas(state, CLOSURE_NOT_READY, CLOSURE_READY)) { - return; /* early out */ - } - - /* The 'acquire' load here matches the 'release' cas in notify_on and - set_shutdown */ - gpr_atm curr = gpr_atm_acq_load(state); - switch (curr) { - case CLOSURE_READY: { - /* Already ready. We are done here */ - break; - } + while (true) { + gpr_atm curr = gpr_atm_acq_load(state); - case CLOSURE_NOT_READY: { - /* The state was not CLOSURE_NOT_READY when we checked initially at the - beginning of this function but now it is CLOSURE_NOT_READY again. - This is only possible if the state transitioned out of - CLOSURE_NOT_READY to either CLOSURE_READY or and then - back to CLOSURE_NOT_READY again (i.e after we entered this function, - the fd became "ready" and the necessary actions were already done). - So there is no need to make the state CLOSURE_READY now */ - break; - } + switch (curr) { + case CLOSURE_READY: { + /* Already ready. We are done here */ + return; + } - default: { - /* 'curr' is either a closure or the fd is shutdown */ - if ((curr & FD_SHUTDOWN_BIT) > 0) { - /* The fd is shutdown. Do nothing */ - } else if (gpr_atm_no_barrier_cas(state, curr, CLOSURE_NOT_READY)) { - /* The cas above was no-barrier since the state is being transitioned to - CLOSURE_NOT_READY; notify_on and set_shutdown do not schedule any - closures when transitioning out of CLOSURE_NO_READY state (i.e there - is no other code that needs to 'happen-after' this) */ + case CLOSURE_NOT_READY: { + if (gpr_atm_no_barrier_cas(state, CLOSURE_NOT_READY, CLOSURE_READY)) { + return; /* early out */ + } + break; /* retry */ + } - grpc_closure_sched(exec_ctx, (grpc_closure *)curr, GRPC_ERROR_NONE); + default: { + /* 'curr' is either a closure or the fd is shutdown */ + if ((curr & FD_SHUTDOWN_BIT) > 0) { + /* The fd is shutdown. Do nothing */ + return; + } else if (gpr_atm_full_barrier_cas(state, curr, CLOSURE_NOT_READY)) { + grpc_closure_sched(exec_ctx, (grpc_closure *)curr, GRPC_ERROR_NONE); + return; + } + /* else the state changed again (only possible by either a racing + set_ready or set_shutdown functions. In both these cases, the closure + would have been scheduled for execution. So we are done here */ + return; } - /* else the state changed again (only possible by either a racing - set_ready or set_shutdown functions. In both these cases, the closure - would have been scheduled for execution. So we are done here */ - break; } } } -- cgit v1.2.3