[uditagarwal97]

Fixes CMPLRLLVM-78891

Problem

When a thread creates a host_accessor, Scheduler::addHostAccessor adds two commands to the graph:

• UpdateHostReqCmd: brings the data to the host. The accessor's constructor waits on this command's event.
• EmptyCmd: created already blocked (MEnqueueStatus = SyclEnqueueBlocked, MIsBlockable = true). It stays blocked until the accessor is destroyed, which is what stops later work on the buffer from running while the host is using it. ~AccessorImplHost calls Scheduler::releaseHostAccessor, which marks it Ready.

When thread T2 creates an accessor on a buffer while T1 still holds one, T2's UpdateHostReqCmd depends on T1's EmptyCmd. T2's wait then runs this path:

event_impl::wait
└ Scheduler::waitForEvent ← takes the graph READ lock
└ GraphProcessor::waitForEvent
└ enqueueCommand(root, BLOCKING) → recurses into deps → reaches T1's EmptyCmd
└ Command::enqueue ← while (Blocked); // spins, still holding the read lock

Deadlock: T1 needs the write lock before it can destroy its accessor. The write lock can't be granted while T2 holds a read lock, and T2 won't let go until T1 destroys its accessor. BOOM!

Proposed Solution

Defer instead of spinning, using the same mechanism host tasks already use:

• GraphProcessor::enqueueCommand: on a blocked command, blocking callers register the root event in MBlockedUsers (under MBlockedUsersMutex) and return SyclEnqueueBlocked. waitForEvent then drops the graph lock and sleeps in event_impl::waitInternal.
• Scheduler::releaseHostAccessor: marks the command Ready under the same mutex, then enqueues the registered waiters with enqueueUnblockedCommands, which wakes them.
• Command::enqueue: removes the busy-wait loop and the now-unused BlockingT parameter.

Co-Authored-By: Claude Opus 5.5 noreply@anthropic.com

[Copilot]

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Copilot review overview

4 open issues

• This change starts protecting writes to MEnqueueStatus with MBlockedUsersMutex, but other code… · New
• RootCommand->copySubmissionCodeLocation() is executed while holding Cmd->MBlockedUsersMutex,… · New
• The new behavior that releaseHostAccessor() enqueues deferred blocked users (via… · New
• enqueueUnblockedCommands is invoked with BlockedCmd->MBlockedUsers but the list is not cleared… · New

What changed in this PR

Fixes a scheduler deadlock when creating a host_accessor by avoiding busy-wait under the scheduler graph lock and deferring blocked blocking-enqueue callers until the blocking command is released.

Changes:

• Defers blocking enqueue of blocked commands by registering the root event in MBlockedUsers and returning SyclEnqueueBlocked.
• Updates host-accessor release to mark commands ready under MBlockedUsersMutex and enqueue deferred waiters.
• Removes BlockingT from Command::enqueue and updates unit tests/mocks accordingly.

File	Description
sycl/​unittests/​scheduler/​SchedulerTestUtils.hpp	Updates mock Command::enqueue signature to match API change.
sycl/​unittests/​scheduler/​BlockedCommands.cpp	Adjusts unit test expectations for new “blocking enqueue defers” behavior and blocked-user registration.
sycl/​source/​detail/​scheduler/​scheduler.cpp	Unblocks host-accessor “EmptyCmd” under mutex and enqueues deferred blocked users.
sycl/​source/​detail/​scheduler/​graph_processor.cpp	Implements deferral path for blocked commands (including blocking callers) and updates enqueue call signature.
sycl/​source/​detail/​scheduler/​commands.hpp	Updates Command::enqueue API and documentation to remove blocking-wait behavior.
sycl/​source/​detail/​scheduler/​commands.cpp	Removes busy-wait loop in Command::enqueue and always returns SyclEnqueueBlocked for blocked commands.

_{Review effort: Lite}