[pbs-devel] [PATCH proxmox-backup v6 19/21] datastore: document s3 backend specific locking restrictions

[Christian Ebner <c.ebner@proxmox.com>]

The requirements are stricter here, since not only must it be avoided
to hold std::sync::Mutex guards for async contexts, but also there
must be consistency between s3 object store, local datastore cache
and in-memory LRU cache.

Signed-off-by: Christian Ebner <c.ebner@proxmox.com>
---
 pbs-datastore/src/lib.rs | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/pbs-datastore/src/lib.rs b/pbs-datastore/src/lib.rs
index 849078a8f..1f7c54ae8 100644
--- a/pbs-datastore/src/lib.rs
+++ b/pbs-datastore/src/lib.rs
@@ -81,6 +81,19 @@
 //! because running these operations concurrently is treated as a feature
 //! on its own.
 //!
+//! For datastores with S3 backend there are further restrictions since
+//! there are 3 types of locking mechanisms involved:
+//! - per-chunk file lock
+//! - chunk store mutex lock
+//! - lru cache mutex lock
+//!
+//! Locks must always be acquired in this specific order to avoid deadlocks.
+//! The per-chunk file lock is used to avoid holding a mutex lock during calls
+//! into async contexts, which can deadlock otherwise. It must be held for the
+//! whole time from starting an operation on the chunk until it is persisted
+//! to s3 backend, local datastore cache and in-memory LRU cache where
+//! required.
+//!
 //! ## Inter-process Locking
 //!
 //! We need to be able to restart the proxmox-backup service daemons, so
-- 
2.47.3



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